The present invention relates to spectrum measuring equipment which can be used with a spectrum analyzer, for instance.
The spectrum analyzer or similar spectrum measuring apparatus employs a prison and diffraction grating, or like dispersing element, whereby light to be measured is split into wave components of respective wavelengths.
FIG. 1 shows in section the structure of a diffraction grating 14, which has about ten to hundreds of equally-spaced-apart minute grooves U per millimeter cut in the surface of sheet glass. When light to be measured Q, which has an optical axis in a plane perpendicular to the grooves U (i.e. in the plane of the paper of the drawing), is incident to the diffraction grating 14, light waves of wavelengths .lambda..sub.1 and .lambda..sub.2, for example, contained in the light Q, are dispersed and reflected in the direction of arrangement of the grooves U in the above-mentioned plane. The angles of dispersion, .theta..sub.1 and .theta..sub.2, of the light waves are dependent upon their wavelengths .lambda..sub.1 and .lambda..sub.2. In the following description the direction of change, D, in the angle of dispersion with wavelength will be referred to as the direction of separation of light, and each angle in the direction D as an angle of separation of light, i.e. an angle of diffraction. Consequently, the direction of light separation D is in the plane of the paper of the drawing FIG. 1.
The quantities of light of the wavelengths .lambda..sub.1 and .lambda..sub.2 thus dispersed or separated, are measured by scanning a photodetector 16 in the direction of light separation D relative to the diffraction grating 14. The wavelength distribution of light is obtained by detecting the levels of the received light signals developed at the positions of respective angles of light separation when the photodetector 16 is scanned in the direction of light separation D. The scanning of the photodetector 16 relative to the diffraction grating 14 in the direction of light separation D may be done by turning the diffraction grating 14 or moving the photodetector 16 in the direction D. It is customary in the art to turn the diffraction grating 14 in the direction of light separation D about a straight line O.sub.L parallel to the grooves U.
Incidentally, the dispersing element such as a diffraction grating has a shortcoming that when the light to be measured Q incident thereto is polarized light, the diffraction efficiency varies with the angle of its plane of polarization, causing a change in the quantities of light of the wavelengths .lambda..sub.1 and .lambda..sub.2 to be dispersed or separated. This phenomenon is commonly referred to as a polarization dependency of the dispersing element.
FIG. 2 shows the polarization dependency characteristic of the diffraction grating. The curve g(.lambda.) represents the diffraction efficiency for the light wave of each wavelength in the case where the plane of polarization of the incident light is parallel to the direction of light separation D, i.e. where the plane of polarization is perpendicular to the grooves U of the diffraction grating 14. The curve f(.lambda.) represents the diffraction efficiency for the light wave of the wavelength in the case where the plane of polarization of the incident light is perpendicular to the direction of light separation D, i.e. where the plane of polarization is parallel to the grooves U of the diffraction grating 14. What is meant by the two curves g(.lambda.) and f(.lambda.) is that when the plane of polarization of light incident to the diffraction grating has turned from the direction parallel to the direction of light separation D to the direction perpendicular thereto, the diffraction efficiency varies from the curve g(.lambda.) to f(.lambda.), that is, the level of diffracted light at each wavelength varies accordingly. As will be seen from FIG. 2, the diffraction grating has no polarization dependency only at a wavelength .lambda..sub.0 , but has the polarization dependent characteristics at other wavelengths. In the case of measuring light emitted from an optical fiber, the influence of variations in the diffraction efficiency of the dispersing element is particularly great, because the plane of polarization of the emitted light has undergone substantial variations according to the state of the optical fiber.
In Japanese Patent Application Laid Open No. 28623/87 there is proposed spectrum measuring equipment which has solved the above problem. The spectrum measuring equipment disclosed in that publication has an arrangement in which the light to be measured is dispersed or separated by a dispersing element, the dispersed light is split by a polarizing element into polarized light components P and S whose planes of polarization are perpendicular to each other. The P and S polarized components are applied to two different photodetectors to obtain electric signals corresponding to the quantities of light of the P and S polarized components. Based on the electric signals, the diffraction efficiencies Ap and Bs of the polarized components P and S in the dispersing element, the loss ratios Cp and Ds of the polarized components P and S in the polarizing element, and the photoelectric conversion efficiencies L1 and L2 of the photodetectors, stored in a memory for each wavelength, are read out for obtaining the absolute power of the light Q through calculation. With this conventional spectrum measuring equipment, it is possible to avoid the influence of the change in the diffraction efficiency due to the difference in angle between the planes of polarization in the dispersing element. To perform this, however, it is necessary to prestore, in the memory, correction data such as the diffraction efficiencies Ap and Bs of the dispersing element for the polarized components P and S, the loss ratios Cp and Ds for the polarized components P and S in the polarizing element, and the photoelectric conversion efficiencies L1 and L2 of the photodetectors. Since the correction data is needed for each wavelength, an appreciable amount of data must be prepared for measurement with high resolution. Further, since the data differs in value with different products, the preparation of such data is time-consuming and hence introduces complexity in the fabrication of equipment. Moreover, since there are cases where the spectrum measurement may sometimes be subject to the influence of variations in the diffraction efficiency according to the values of the correction data stored in the memory, the polarization dependency of the dispersing element cannot always be eliminated.